X-ray imaging is the mainstay of diagnostic radiology, with over 20 million radiographic procedures performed in Canada alone each year. Although x-ray technologies have been under development for over 100 years, their cost, use and potential for diagnosis continue to accelerate. Due to the health risks associated with exposure to radiation and the risks due to inconclusive or misleading diagnoses, technical excellence in the design and maintenance of x-ray medical imaging systems is critical to achieving high-quality images and medical care. In x-ray imaging, image quality is a balance between system performance and patient radiation dose. Unfortunately, not all systems—both new and old—provide patients with the benefits of the highest possible image quality consistent with a specified radiation exposure. This is particularly true for some of the emerging lower-cost digital technologies that are beginning to have a substantial impact on the practice of radiology.
The use of sub-optimal equipment has two impacts on patient well being. The first is unnecessary exposures to ionizing radiation. This risk is managed in many jurisdictions across Canada, the US and Europe by implementing maximum allowable exposure levels for standardized radiographic procedures under specified conditions. Of potentially greater risk is that of missed or misleading diagnoses due to sub-optimal image quality. Image signal-to-noise ratio (SNR) is related to both x-ray exposure levels and the ability of an imaging system to extract the best possible image SNR from the radiation incident on the imaging detector. A poor image SNR prevents the detection of low-contrast lesions, such as small tumours, and it is therefore critical that x-ray imaging systems be designed and maintained to produce the best possible image SNR consistent with acceptable levels of radiation exposure.
Medical facilities routinely monitor image quality using the best practices and equipment currently available. These include measurements of “line-pair” test objects to determine spatial resolution, low-contrast test objects to determine “detectability,” anthropomorphic test phantoms and others. While these are subjective measures with little quantitative value, they are the best presently available and hence are in wide-spread use. In addition, they provide no specific information regarding the “dose efficiency” of the imaging system. Systems with poor dose efficiency may be able to produce high-quality images, but require increasing the radiation dose delivered to the patient. With many of the new digital technologies, it has become even more difficult for equipment users to be aware of these shortcomings. While the onus is on the user to make a wise purchase decision and to ensure proper equipment maintenance, there is no instrumentation available to the non-expert user to assess the performance and dose efficiency of their equipment.
The scientific community has generally adopted use of both the modulation transfer function (MTF) and detective quantum efficiency (DQE) as the most appropriate measures of system performance. The MTF is expressed as a function of spatial frequency and describes spatial resolution. The DQE is also expressed as a function of spatial frequency and is a measure of system “dose efficiency” and therefore risk to the patient. A high-quality imaging system will always have excellent MTF and DQE parameters. The DQE of an ideal system is unity for all spatial frequencies of importance; however, most systems range between 0.1 and 0.5, and all too often even less. The DQE differs from one manufacturer to another, and may vary with system design, exposure level, system age, and level of servicing. Since DQE is inversely proportionate to dose, in many cases by optimizing x-ray parameters, there exist opportunities to achieve patient dose reductions by factors of 2 to 10 without compromising image SNR.
The practical significance of the DQE is generally accepted by major vendors. In the United States, the Food and Drug Administration (FDA) now requires submission of both MTF and DQE documentation before approval is issued for new radiographic devices. However, the FDA does not verify manufacturers' claims and it is up to the end user to ensure acceptability of the equipment. The DQE is thought to be such an important metric by the community in general that standards are being developed by scientists and engineers in both academia and industry around the world, such as Task Group #16 of the American Association of Physicists in Medicine and Working Group 33 of the International Electrotechnical Commission (IEC 62220-1). These standards are attempting to establish consistency in DQE measurements to enable comparisons between different imaging systems and manufacturers and quantitative interpretation of DQE values.
A common expression for DQE as a function of spatial frequency is:
                              DQE          ⁡                      (            u            )                          =                                            MTF              2                        ⁡                          (              u              )                                                          XQ              o                        ⁢                                          NPS                ⁡                                  (                  u                  )                                            /                              d                2                                                                        (                  Equation          ⁢                                          ⁢          1                )            
where                u is spatial frequency, often expressed in units of cycles/mm;        MTF(u) is the measured MTF;        X is the measured incident free-air exposure in Roentgen (R), or air KERMA (kinetic energy released in medium) in Gray (Gy);        Qo relates the input exposure or air KERMA to an associated number of incident x-ray quanta per mm2;        NPS(u) is the measured Wiener noise power spectrum (NPS), and        d is the mean dark-subtracted pixel value in open-field images used to calculate the NPS.        
A measurement of the DQE is achieved by determining each of these parameters and solving the above Equation 1.
There are, however, four serious technical issues that restrict widespread use of the DQE, such that measurements are performed only by a few experts, and only in laboratory or special test environments:
1. The first is the time and effort required by a non-expert to acquire the necessary expertise in DQE physics. This includes becoming an expert in x-ray physics, Fourier methods and the theoretical basis of the DQE and measurement techniques.
2. The second is the time and effort required to create and validate a facility to measure the quantities required to calculate the DQE. Guidelines have been established (IEC 62220-1) that help describe how such a facility should be constructed. Great care must be taken to ensure measurements are not subject to inadequate considerations that could result in erroneous DQE results. These include but are not limited to x-ray scatter, poor design of components used in the measurement and inadequate monitoring of x-ray intensity fluctuations.
3. The third is developing the necessary software required to calculate the DQE from acquired images and measured data. Software is freely available to calculate certain components of the DQE, such as the modulation transfer function (MTF), but no validated, readily available software currently exists to complete the DQE calculation.
4. The fourth is that each DQE measuring facility must be validated to ensure accuracy and consistency with other facilities. This validation is very difficult as sites contain different x-ray equipment, and there is no generally accepted test object to calibrate the facility against a standard or enable inter-site comparisons. Rather, validation can only be performed by comparing results obtained using a particular imaging system with results obtained elsewhere using a similar imaging system. A comprehensive validation, under a range of conditions or for new imaging systems is extremely difficult.
U.S. Pat. No. 6,521,886 (“'886”), “Method of monitoring changes in the detective quantum efficiency of an x-ray detector”, describes methods and apparatus to allow determination of changes in DQE relative to an initial standard. A “portable” DQE measurement facility involving a working table surface mounted on wheels is described. Some of the equipment required to measure the DQE is contained on the moveable table and can be wheeled out of the way when not in use, reducing the requirement for a dedicated DQE facility. However the system remains very cumbersome, comprising a complete lab bench with wheels. In addition, a trained physicist is still required to measure the DQE using conventional manual instrumentation and techniques. Further, '886 notes that the DQE is proportional to the ratio R(u) where
                              R          ⁡                      (            u            )                          =                                            MTF              2                        ⁡                          (              u              )                                                          NPS              ⁡                              (                u                )                                      /                          d              2                                                          (                  Equation          ⁢                                          ⁢          2                )            
where MTF(u) is the system MTF expressed as a function of spatial frequency u, d is the average pixel value in an image acquired with a uniform x-ray exposure and NPS(u) is the corresponding image noise-power spectrum at that exposure. Thus, if all other factors remain unchanged, measured changes in either MTF(u) or NPS(u)/d2 may indicate a change in R(u) and therefore a change in DQE. However, no methods are taught in '886 for determination of actual DQE values of the x-ray detector. Rather, methods are restricted to monitoring only time-course changes in the DQE relative to an initial arbitrary reference value obtained on the same unit. While measurement of R(u) is simpler than a measurement of DQE(u) and does not require a DQE measurement facility, it does not allow determination of actual DQE values of an x-ray detector that could be used to compare the performance characteristics of an x-ray imaging system to theoretical expectations, specific industry standards, or to other x-ray imaging systems made by the same or different manufacturers.